1. Field of the Invention
The present invention relates to a process for preparing caprolactam. More particularly, the present invention relates to a process for producing caprolactam from 5-formylvaleric acid and its ester using water, an alcohol or a mixture as a solvent and using a noble metal catalyst supported by a Group II or IV oxide.
2. Description of the Prior Art
Caprolactam is an important raw material for nylon 6 and nylon 6 can be obtained from subjecting caprolactam to open ring polymerization. Nylon 6 has a great variety of uses, and it is mainly used for fibrous material and engineering plastics. As a fibrous material, nylon 6 can be made into clothes, carpets, tire cords and filter cloths. As an engineering plastic, nylon 6 can be made into component parts of automobiles, machines and electronic equipment.
Conventionally, caprolactam is prepared from cyclohexane oxime, as disclosed in U.S. Pat. Nos. 4,268,440 and 5,304,643. This process involves the Beckmann rearrangement of cyclohexanone oxime to obtain caprolactam. The disadvantages of this process are the cost of investment is too high and a great amount of undesirable ammonium sulfate waste is produced. Therefore, many other new processes have been established.
U.S. Pat. No. 4,730,040 uses methyl 5-formylvalerate as a starting material to produce caprolactam. In this process, methyl 5-formylvalerate is first hydrolyzed in the presence of an acidic catalyst at 30.degree. to 200.degree. C. to 5-formylvaleric acid, which is then reacted with excess ammonia and hydrogen in the presence of Raney nickel or Raney cobalt as a hydrogenation catalyst at 50.degree. to 150.degree. C. under superatmospheric pressure to form 6-aminocaproic acid. After the unreacted ammonia and hydrogen gas are removed, the aqueous mixture is heated to 150.degree. to 370.degree. C. under 100 bar to induce dehydrolyzation and cyclization of 6-aminocaproic acid to form caprolactam. Such process is very complicated and the yield is not high enough.
U.S. Pat. Nos. 4,730,041 and 4,731,445 use methyl 5-formylvalerate as a starting material and an alkanol as a solvent to produce caprolactam. First, methyl 5-formylvalerate is reacted with excess ammonia and hydrogen in the presence of an alkanol as a solvent and in the presence of a magnesium silicate-supported nickel catalyst at 40.degree. to 130.degree. C. under superatmospheric pressure to form methyl 6-aminocaproate. After the unreacted ammonia and hydrogen gas are removed, methyl 6-aminocaproate is converted into 6-aminocaproic acid and then formed to caprolactam by dehydrolyzation and cyclization or by direct de-alcoholization and cyclization. Although the process does not involve the hydrolyzation procedure of 5-formylvalerate, it suffers from the disadvantage that the starting material has a very low concentration (only 10%) and a great amount of alkanol should be separated and recycled.
Japanese Patent No. 29148 (1968) uses ammonia water as a solvent to convert 5-formylvalerate into caprolactam by direct cyclization in the presence of Raney nickel at 230.degree. C. under 150 bar. This process suffers from the disadvantage that the yield is too low and fluctuates very greatly, and the pressure employed is too high, which can not meet the requirement for commercial production.
European Patent Application EP 729944 involves first contacting 5-formylvaleric acid with ammonia, contacting the resultant product with hydrogen in the presence of a hydrogenation catalyst to form 6-aminocaproate, and heating the resultant product at 200.degree. to 350.degree. C. for de-alcoholization and cyclization to produce caprolactam. Although such process employs lower pressure, an additional amination step is needed, thus making the process complicated.
U.S. Pat. No. 4,963,672 discloses another process for preparing caprolactam, wherein the 5-formylvaleric ester is reacted with excess ammonia and hydrogen gas in the presence of a ruthenium catalyst supported by alumina under 40 to 100 bar at from 80.degree. to 140.degree. C. for conversion into methyl 6-aminocaprocate, the pressure is decreased to standard pressure, hydrogen is removed, and ammonia is replaced with xylene. After that, the pressure is increased to 70 to 100 bar and the resultant mixture is heated to 230.degree. to 350.degree. C. to convert methyl 6-aminocaproate into caprolactam by de-alcoholization and cyclization. This process is still complicated, and has the disadvantage that the pressure is increased, then decreased, and then increased again, and a great amount of xylene should be removed.